On a surface of an optical component constituting an optical instrument, an antireflection film is formed to improve light transmittance.
When, in air, a low refractive index material of which refractive index nc isnc=√ng  (Formula 1)to the refractive index ng of a base material is coated at an optical film thickness of λ/4 to light having a wavelength λ, the refractive index theoretically becomes zero.
A general antireflection film is formed by vacuum depositing a material having the refractive index lower than that of a substrate. As a low refractive index material, magnesium fluoride (MgF2) having nd=1.38 is in broad use. Here, nd is the refractive index to light having a wavelength of 587 nm.
When magnesium fluoride (nd=1.38) is disposed on an optical glass BK7 (nd=1.52) at an optical film thickness of λ/4, residual reflectance of 1.26% is generated.
In this case, to nullify the reflectance, the refractive index nc is necessary to benc=√nd(BK7)=√1.52=1.23  (Formula 2).
As an antireflection film of an optical element necessary to have a lower reflection effect, not the single layer film but a multilayer film formed by alternately laminating a high refractive index film and a low refractive index film is used. Also in this case, as the uppermost layer on an air side, a low refractive index material is important.
On the other hand, an attempt to make the refractive index lower by forming a porous film is broadly conducted. When materials A and B having different refractive indices are mixed at a ratio of p:1−p, apparent refractive index n is represented byn=nA×p+nB×(1−p)=nB−p×(nB−nA)  (Formula 3).Herein, p represents the porosity.
It is suggested to be advantageous to form a porous film with a gas (usually, air) having the refractive index 1 to obtain a low refractive index. Herein, when a material A is air, nA≈1, accordingly, formula (3) becomesn=nB−p×(nB−1)  (Formula 4).This is the refractive index exhibited by a material having a bulk refractive index nB when the porosity thereof is p.
When magnesium fluoride (nd=1.38) is used as the low refractive index material to obtain a porous film having the apparent refractive index n=1.23, the porosity of about 40% is necessary.
Examples of the materials having the refractive index lower than that of magnesium fluoride (MgF2) include aluminum fluoride (AlF3) and sodium fluoride (NaF). Although these are known in several crystal forms, sodium hexafluoroaluminate (3NaF·AlF3) of which nd is 1.339 can be cited as a typical one.
As a method for preparing a porous film, not a dry process according to vacuum deposition but a wet process is effective. In the case of wet process, after a coating material is dissolved or dispersed in a solvent, various coating methods can be used to deposit, and accordingly, there is an advantage that a porous film tends to be readily obtained.
On the other hand, examples of methods where magnesium fluoride is prepared according to the wet process include methods illustrated below. Japanese Patent Application Laid-Open No. 59-213643 and M. Tada et al., J. Mater. Res., Vol. 14, No. 4, April, 1999, page 1610 to 1616 discuss methods where magnesium fluoride is prepared according to a thermal disproportional reaction. After a fluorine-containing magnesium compound or a precursor thereof is coated on a substrate, magnesium fluoride is produced by the thermal disproportional reaction. In both cases, the refractive index is around 1.39, that is, only a value of bulk magnesium fluoride is illustrated.
In I. Sevonkaev et al., J. Colloid and Interface Science, 317 (2008), page 130 to 136, a method of preparing a mixed system of magnesium fluoride and sodium fluoride is discussed. However, a mixture of magnesium fluoride and sodium fluoride prepared according to a wet process is not sufficient because of deliquescency.
Thus, according to conventional technologies, magnesium fluoride prepared according to the wet process that uses the thermal disproportional reaction is insufficient in characteristics from the viewpoint of a low refractive index. In addition, a mixed fluoride of magnesium fluoride and sodium fluoride prepared according to the wet process cannot obtain practically sufficient characteristics in deliquescency. Furthermore, in magnesium fluoride prepared according to the wet process, a sufficient mechanical strength is not obtained.